LIDO uses the "singapore model" to calculate this value. The model mathematically quantifies both the velocity and direction differences between two altitudes. Generally 2000 feet above and below the planned altitude are used. The value presented is a value in knots/1000 ft.
The challenge is that this is an instantaneous value at a specific waypoint and is only an indicator of the potential for CAT.
No studies have been done to correlate the value and CAT.
There is also no ability to relate the shear value and the level of potential turbulence i.e. a value of 3 is not moderate or 5 severe, there is NO established relationship.
I have flown using these values for years and seen values of 5 or 6 and been smooth and had 0 and been in moderate turbulence.
The expectation is that you look at this value and then couple it with SIG WX charts and PIREPS to formulate the best decision.
Also be aware that there are several SIG WX models that produce the SIG charts and different models produce quite different predictions.